1. Field of the Invention
The present disclosure relates to a fuel injector with adjustable-metering servo valve for an internal-combustion engine.
2. Description of Related Art
As is known, the servo valve of an injector in general comprises a control chamber of the usual control rod of the nozzle of the injector. The control chamber is provided with an inlet hole in communication with a pipe for the fuel under pressure and a calibrated hole for outlet or discharge of the fuel, which is normally closed by a shutter controlled by the armature of an electromagnet. The stroke or lift of the armature determines the readiness of the response of the servo valve both for opening and for closing so that it should be as small as possible. Said stroke also determines the section of passage of the fuel through the discharge hole, so that it should to be as wide as possible within the limits of the section of the outlet hole of the control chamber. Consequently, it is necessary to adjust the stroke of the armature and/or of the shutter accurately.
Servo valves are known with the shutter separated from the armature, the stroke of which is defined on one side by the arrest against the shutter in a position for closing the discharge hole. In a known servo valve, the armature is guided by a sleeve, one end of which forms the element for arrest of the stroke of the armature in the direction of the core of the electromagnet. The sleeve is in turn fixed in a cavity of the casing in a position, with respect to the valve body, such as to define the range of the stroke of the armature for opening of the discharge hole. The adjustment of the stroke of the armature is obtained by using at least one removable shim, set between the sleeve and the core of the electromagnet, in order to define the stroke of the armature, and at least another removable shim set between the sleeve and the valve body in order to define the gap of the armature.
The aforesaid shims can be chosen from among classes of calibrated and modular shims. For technological reasons and for economic constraints of feasibility, said shims can vary from one another by an amount not less than the machining tolerance, for example 5 micrometers (μm). The operation of adjustment of the stroke of the armature by discrete amounts with a tolerance of 5 μm is, however, relatively rough, so that it is often impossible to keep the flow rate of the injector within the very narrow limits required by modern internal-combustion engines. Consequently, the operation of adjustment is complicated, requiring different successive attempts of approximation, each of which involves dismantling and the re-assembly of part of the injector. In any case, adjustment on the one hand requires a considerable amount of time on the part of a skilled operator, and on the other hand is often imperfect on account of the aforesaid discrete amounts.
Also known from the document EP-A-0 890 730 is a servo valve, in which the sleeve for guiding the armature is provided with a flange that is relatively deformable to bending loads. The same sleeve is moreover provided with a thread for fixing it in the cavity of the casing, independently of the valve body. The position of the flange is adjusted, by means of shims, in discrete positions of a given interval, for example 5 μm. Subsequently, by screwing the sleeve by applying a calibrated tightening torque, the flange is deformed so as to enable a fine adjustment to be made.
In the known servo valves described above, the shutter is subjected on the one hand to the axial thrust exerted by the pressure of the fuel in the control chamber and on the other hand to the action of the axial thrust of a spring that is pre-loaded so as to overcome the thrust of the pressure when the electromagnet is not excited. The spring then presents characteristics and dimensions such as to be able to exert a considerable axial thrust, for example in the region of 70 Newtons (N) for a pressure of the fuel of 1800 bar. Upon excitation of the electromagnet, the armature is displaced and comes to stop against a fixed element, in a position such as to enable a residual minimal gap with respect to the core of the electromagnet, in order to optimize prompt reaction of the servo valve to de-excitation of the electromagnet.
In order to reduce pre-loading of the spring for closing the shutter, a servo valve has recently been proposed, in which the fuel under pressure no longer exerts an axial action, but acts in a radial direction on the support of the shutter, so that the action of the pressure of the fuel on the shutter is substantially balanced. The action of the spring and that of the electromagnet can thus be of a lower value. Also in this known servo valve, it has been proposed to adjust the stroke of the armature by means of one or more shims, set between a flange of the core of the electromagnet and a shoulder of the casing of the injector. Installation of the shims requires, however, a relatively long time, so that the injector is rather costly to make.